| AVS 60th International Symposium and Exhibition | |
| Spectroscopic Ellipsometry Focus Topic | Thursday Sessions |
| Session EL+AS+EN+PS+SS+TF-ThM |
| Session: | Spectroscopic Ellipsometry for Photovoltaics and Instrument Development |
| Presenter: | P. Koirala, The University of Toledo |
| Authors: | P. Koirala, The University of Toledo J. Chen, The University of Toledo X. Tan, The University of Toledo R.W. Collins, The University of Toledo |
| Correspondent: | Click to Email |
In-situ real-time spectroscopic ellipsometry (RT-SE) from the film side has been applied along with ex-situ spectroscopic ellipsometry through the glass (TG-SE) toward the analysis of the different stages of CdS/CdTe solar cell fabrication in the superstrate configuration. The RT-SE studies of the CdS and CdTe layers deposited on transparent conducting oxide (TCO) coated glass superstrates provide information on the CdS growth, its surface roughness evolution, as well as overlying CdTe interface formation and bulk layer growth. Such information is very useful for developing a realistic optical model for the as-deposited layer structure in TG-SE mapping analysis over the full 15 cm x 15 cm superstrate area. In the mapping analysis, a mask is used to measure all 256 points where 0.125 cm2 area dot cells are to be made. Because the as-deposited superstrate/film-structure undergoes additional processing steps, however, in order to complete the solar cell devices, three sets of TG-SE data maps are collected that characterize the superstrate/film-structure in the (i) as-deposited, (ii) CdCl2-treated, and (iii) back-contact patterned states. With the optical database that has been established for both the as-deposited and CdCl2 treated CdS and CdTe, each of the three TG-SE data maps has been analyzed based on an optical model deduced from RT-SE studies of the CdS and CdTe depositions. By using these SE techniques, we have been able to quantify the spatial dependence of the changes that occur in the superstrate/film-structure as a result of the different processing steps. In order to corroborate the layer structure determined by TG-SE, comparisons with cross-sectional transmission electron microscopy (XTEM) have been performed for selected states of the superstrate/film-structure and for selected locations. The results have been found to validate the overall RT-SE and TG-SE approach and analysis results. Finally, the layer parameters as determined from the TG-SE analyses have been correlated statistically with the device performance of the 256 dot cells fabricated over the final 15 cm x 15 cm superstrate/film-structure. The resulting correlations expedite solar cell optimization.